Presently, light guide based near-to-eye displays utilizing diffractive exit pupil expanders (EPE) are suitable for generation of virtual images to a fixed viewing distance. Because of the use of transparent light guides, such displays can be operated in a see-through mode to enable the generated images to be viewed simultaneously with the physical background. This enables a direct route for implementation of augmented reality applications where virtual information is overlaid on top of the physical scenery. However, for simultaneous viewing of physical and virtual objects their convergence and accommodation distance should to match. This means that the display system should allow adjustable viewing distance to bring the virtual information to coincide with physical objects.
Near-to-eye displays have to be able to create multiple viewing distances simultaneously. Unfortunately, for near-distance objects, each object point is replicated in the exit pupil expander (EPE) plate, which is seen as pixel blurring in the virtual display. For example, if the imaging optics of the near-to-eye display is designed to provide an input image at a finite viewing distance, each display pixel will consist of a range of ray angles. When coupled into a planar exit pupil expander (EPE) the whole range of incident ray angles will emerge from all the ray-interception points along the out-coupling grating. Thus, a single display pixel will appear to be replicated or blurred when observed through the exit pupil expander (EPE). The limitation of infinite viewing distance may be a problem for near-sighted people. In particular, in mobile applications compact eyeglass type near-to-eye displays are preferred that are not well suited for wearing with regular eyeglasses. Furthermore, the limitation of infinite image distance can lead to significant reduction in the viewing experience and limit the range of potential applications, such as stereoscopic 3D (3-dimensional) viewing.